Method and system for subscriber line interface circuit

ABSTRACT

A subscriber line interface circuit apparatus includes a linefeed circuit and a subscriber line control circuit (SLCC). In an embodiment, the linefeed circuit includes a signal conversion circuit which provides a differential mode signal and a common mode signal in response to at least a tip signal and a ring signal from the subscriber loop. The linefeed circuit includes a tip control circuit and a ring control circuit. In an embodiment, the SLCC is provided in a single integrated circuit chip and is coupled to the linefeed circuit which isolates the SLCC from the tip or ring signals. The SLCC includes a first and a second differential mode inputs for receiving the differential mode signal, and a common-mode input for receiving the common-mode signal. In an embodiment, the SLCC also provides various tip control signals and ring control signals to the tip control circuit and the ring control circuit, respectively.

BACKGROUND OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides a method and device for anintegrated circuit for telecommunication applications. Merely by way ofexample, the invention has been applied to subscriber line controlcircuits for interfacing with digital equipment in a central office. Butit would be recognized that the invention has a much broader range ofapplicability. For example, embodiments of the invention can findapplication in communication networks such as cable television networks,fiber optic, Ethernet port interface to the Internet, VoIP, and wirelesslocal loop, etc.

In a Plain Old Telephone Service (POTS) system, several functions areneeded for telephony signaling, subscriber line supervision, andtelephone equipment operations. These functions include battery feed,over voltage protection, ringing, supervision, codec, hybrid, andtesting, etc. These electronic functions are collectively known by theacronym BORSCHT. BORSCHT circuitry is often found on atelecommunications network line card for implementation of a standardPOTS telephone interface for digital equipment in a central office. Withthe advancement of integrated circuit technology, BORSCHT circuitry canbe integrated into a chipset. For example, a line card may include aSLIC, or a subscriber line interface circuit, and a CODEC circuit. In aconventional design, the SLIC typically handles the analog functions anddrives the telephone lines and usually include high voltage devices andcircuits, whereas the CODEC typically handles digital signal processingand interface functions. Some conventional line cards may include a SLICintegrated circuit and a CODEC integrated circuit. A conventional linecard may also include certain discrete components such as switchingrelays, large capacitors, etc. These conventional techniques have manylimitations.

From the above, it is seen that an improved technique for subscriberline interface circuit is desired.

BRIEF SUMMARY OF THE INVENTION

According to an embodiment of the invention, in a subscriber lineinterface circuit apparatus for a subscriber loop, a linefeed circuit incombination with subscriber line control circuitry (SLCC) is providedwherein a first node is coupled to a first signal line and a second nodeis coupled to a second signal line to receive a tip signal and a ringsignal, respectively, from the subscriber loop, with the improvementthat the linefeed circuit includes a unique signal conversion circuitwhich is coupled to the first node and the second node to receive thetip signal and the ring signal, respectively and which provides both adifferential mode signal and a related common mode signal in response toat least the tip signal and the ring signal so the SLCC circuitry neednot derive such signals. In a specific embodiment, the common modesignal is associated with a sum of the tip signal and the ring signal,and the differential mode signal is associated with a difference betweenthe same tip signal and ring signal.

In a specific embodiment, the SLCC includes multiple control signaloutputs for driving amplifiers associated with the tip and ring controlcircuits, in addition to an amplifier circuit for receiving the voiceband or audio band signal from the linefeed circuit, an impedancematching circuit which is coupled to the amplifier circuit, a hybridcircuit coupled to the impedance matching circuit, and a codec which iscoupled to the hybrid circuit. In a specific embodiment, the linefeedcircuit includes discrete components, whereas in another embodiment, thelinefeed circuit is implemented as an integrated circuit.

According to a specific embodiment, the present invention provides asubscriber line interface circuit apparatus for a subscriber loop. Theapparatus includes a linefeed circuit and a subscriber line controlcircuit (SLCC). The SLCC is provided in a single integrated circuit chipand is coupled to the linefeed circuit. In an embodiment, the linefeedcircuit includes a first node coupled to a first signal line and asecond node coupled to a second signal line to receive a tip signal anda ring signal, respectively, from the subscriber loop. The linefeedcircuit includes a signal conversion circuit which is coupled to thefirst node and the second node to receive the tip signal and the ringsignal, respectively. The signal conversion circuit provides adifferential mode signal and a common mode signal in response to atleast the tip signal and the ring signal. In a specific embodiment, thecommon mode signal is associated with a sum of the tip signal and thering signal, and the differential mode signal is associated with adifference between the tip signal and the ring signal. The linefeedcircuit includes a filter circuit which is coupled to the subscriberloop and provides voice band or audio band signals to the SLCC. In someembodiments, audio band includes a frequency range of about 20 Hz to20,000 Hz, and voice band has a frequency range of about 300 Hz to 3,400Hz. The linefeed circuit also includes a tip control circuit and a ringcontrol circuit. The tip control circuit is coupled to the first nodeand provides a tip output signal to the first signal line in response toa first tip control current signal and a second tip control currentsignal. The ring control circuit is coupled to the second node andprovides a ring output signal to the second signal line in response to afirst ring control current signal and a second ring control currentsignal.

In a specific embodiment, a subscriber line control circuit (SLCC) isprovided in a single integrated circuit chip. In an embodiment, the SLCCis coupled to the linefeed circuit and is isolated from the first andsecond signal lines. In an embodiment, the linefeed circuit isolates theSLCC from the first signal line and the second signal line. In otherwords, SLCC does not sense the tip signal or the ring signal accordingto embodiments of the present invention. In contrast, the SLCC receivesa differential mode signal and a common mode signal, and can berelatively more isolated from the tip and ring signals. In a specificembodiment, the SLCC includes a first and a second differential modeinputs and a common mode input. The differential mode inputs receive thedifferential mode signal from the linefeed circuit. The common modeinput receives the common mode signal from the linefeed circuit.

In an embodiment, the SLCC also includes an analog to digital convertercircuit which is coupled to the common mode input and the differentialmode inputs. The SLCC also includes an analog control circuit which iscoupled to the common mode input and the differential mode inputs andprovides the individual tip and ring control signals in response to atleast the common mode signal and the differential mode signal, withoutproviding DC common mode control of the Tip and Ring signals. Thesecontrol signals are described below. The SLCC includes an amplifiercircuit for receiving the voice or audio band signals and a codeccircuit coupled to the amplifier circuit for providing coding anddecoding functions for the voice or audio band signals. The SLCC alsoincludes a first tip control output and a second tip control outputcoupled to the analog control circuit and provides the first tip controlcurrent signal and the second tip control current signal, respectively.The SLCC includes a first ring control output and a second ring controloutput coupled to the analog control circuit and provides the first ringcontrol current signal and the second ring control current signal,respectively.

In a specific embodiment, the subscriber line interface circuitapparatus described above includes a signal conversion circuit thatincludes first transistor and a second transistor. A first terminal ofthe first transistor is coupled to a resistor connected to the firstnode, a second terminal of the first transistor is coupled to a resistorconnected to the second node, a third terminal of the first transistoris coupled to a first differential mode signal line, a first terminal ofthe second transistor is coupled to a resistor connected to the secondnode, a second terminal second transistor is coupled to a resistorconnected to the first node, and a third terminal of the secondtransistor is coupled to a second differential mode signal line. Thefirst and second differential mode signal lines provide the differentialmode signal. In an embodiment, the signal conversion circuit alsoincludes a first resistor and a second resistor. A first terminal of thefirst resistor is coupled to the first node, a second terminal of thefirst resistor is coupled to the common mode signal, a first terminal ofthe second resistor is coupled to the second node, and a second terminalof the second resistor is coupled to the common mode signal.

In a specific embodiment of the subscriber line interface circuitapparatus described above, the tip control circuit includes a first tipcontrol bipolar transistor and a second tip control bipolar transistor.An emitter terminal of each of the first tip control bipolar transistorand the second tip control bipolar transistor is in communication withthe first tip control current signal and the second tip control currentsignal, respectively. In an embodiment, a base terminal of the first tipcontrol transistor is in communication with a base terminal of thesecond tip control transistor. In a specific embodiment, the ringcontrol circuit includes a first ring control bipolar transistor and asecond ring control bipolar transistor. An emitter terminal of each ofthe first ring control bipolar transistor and the second ring controlbipolar transistors is in communication with the first ring controlcurrent signal and the second ring control current signal, respectively.In an embodiment, a base terminal of the first ring control transistoris in communication with a base terminal of the second ring controltransistor. In a specific embodiment, the linefeed circuit comprisesdiscrete components. In another embodiment, the linefeed circuit isimplemented in an integrated circuit.

According to another embodiment of the invention, an integrated circuitapparatus is provided for controlling a subscriber loop. The integratedcircuit apparatus includes a common mode input and a first and a seconddifferential mode inputs. The common mode input receives a common modesignal, and the first and second differential mode inputs receive adifferential mode signal. In a specific embodiment, the common modeinput signal is related to a sum of a tip signal and a ring signal ofthe subscriber loop. In an embodiment, the differential mode signal isrelated to a difference between the tip signal and the ring signal ofthe subscriber loop. In a specific embodiment, each of the common modesignal and the differential mode signal is generated outside theintegrated circuit apparatus. The integrated circuit apparatus isisolated from the tip signal and the ring signal. In other words, theintegrated circuit apparatus does not receive a sensed tip signal or asensed ring signal. The integrated circuit apparatus includes an analogto digital converter circuit which is coupled to the common mode inputand the differential mode inputs. The integrated circuit apparatusincludes an analog control circuit which is coupled to the common modeinput and the differential mode inputs and provides individual tip andring control signals in response to at least the common mode signal andthe differential mode signal, without providing DC common mode controlof the Tip and Ring signals. These control signals include thefollowing:

-   -   1. A first tip control current signal and a second tip control        current signal provided by a first tip control output and a        second tip control output, respectively, which are coupled to        the analog control circuit;    -   2. A first ring control current signal and a second ring control        current signal provided by a first ring control output and a        second ring control output, respectively, which are coupled to        the analog control circuit;

In an embodiment, the integrated circuit apparatus also includes anamplifier circuit for receiving the voice or audio band signal,respectively, an impedance matching circuit couple to the amplifier, ahybrid circuit coupled to the impedance matching circuit, and a codeccoupled to the hybrid circuit. In an embodiment, the analog controlcircuit also includes an analog voltage feedback circuit, a currentfeedback circuit, and a summation circuit.

According to an alternative embodiment of the invention, a subscriberline interface circuit apparatus is provided for a subscriber loop. Theapparatus includes a linefeed circuit and a subscriber line controlcircuit. The linefeed circuit provides a differential mode signal and acommon mode signal in response to at least a tip signal and a ringsignal from the subscriber loop. The linefeed circuit also provides atip output signal and a ring output signal to the subscriber loop inresponse to one or more ring control signals and one or more tip controlsignals. The subscriber line control circuit is coupled to the linefeedcircuit for receiving the differential mode signal and the common modesignal from the linefeed circuit. The subscriber line control circuitprovides the one or more ring control signals and the one or more tipcontrol signals to the linefeed circuit. In an embodiment, the one ormore ring control signals include at least a ring control currentsignal, and the one or more tip control signals include at least a tipcontrol current signal.

In a specific embodiment, the linefeed circuit includes a first nodecoupled to a first signal line and a second node coupled to a secondsignal line to receive a tip signal and a ring signal, respectively,from the subscriber loop. The linefeed circuit includes a signalconversion circuit which is coupled to the first node and the secondnode to receive the tip signal and the ring signal, respectively. Thesignal conversion circuit provides a differential mode signal and acommon mode signal in response to at least the tip signal and the ringsignal. In a specific embodiment, the common mode input signal isassociated with a sum of the tip signal and the ring signal, and thedifferential mode input signal is associated with a difference betweenthe tip signal and the ring signal. The linefeed circuit includes afilter circuit which is coupled to the signal conversion circuit andprovides voice band or audio band signals. The linefeed circuit alsoincludes a tip control circuit and a ring control circuit. The tipcontrol circuit is coupled to the first node and provides a tip outputsignal to the first signal line in response to a first tip controlcurrent signal and a second tip control current signal. The ring controlcircuit is coupled to the second node and provides a ring output signalto the second signal line in response to a first ring control currentsignal and a second ring control current signal. In an embodiment, asubscriber line control circuit (SLCC) is provided in a singleintegrated circuit chip. In a specific embodiment, the SLCC is coupledto the linefeed circuit and is isolated from the first and second signallines. In other words, the SLCC does not sense the tip signal or thering signal. The SLCC includes a first and a second differential modeinputs and a common mode input. The differential mode inputs and commonmode input receive the differential mode signal and the common modesignal, respectively, from the linefeed circuit.

In a specific embodiment, the SLCC includes an analog to digitalconverter circuit which is coupled to the common mode input and thedifferential mode inputs. The SLCC also includes an analog controlcircuit which is coupled to the common mode input and the differentialmode inputs. The analog control circuit provides the individual tip andring control signals in response to at least the common mode signal andthe differential mode signal, without providing DC common mode controlof the Tip and Ring signals. These control signals are described below.The SLCC includes a first and a second tip control outputs coupled tothe analog control circuit and provides the first and second tip controlcurrent signals, respectively. The SLCC also includes a first and asecond ring control outputs coupled to the analog control circuit andprovides the first and second ring control current signals,respectively. The SLCC includes an amplifier circuit for receiving thevoice band or audio band signal, respectively, from the linefeedcircuit, an impedance matching circuit which is couple to the amplifier,a hybrid circuit coupled to the impedance matching circuit, and a codecwhich is coupled to the hybrid circuit. In a specific embodiment, thelinefeed circuit includes discrete components, whereas in anotherembodiment, the linefeed circuit is implemented in an integratedcircuit.

In yet another embodiment, the invention provides a subscriber lineinterface circuit apparatus for a subscriber loop. The apparatusincludes a linefeed circuit and a subscriber line control circuit(SLCC). The linefeed circuit includes a first node and a second node.The first node is coupled to a first signal line to receive a tip signalfrom the subscriber loop, and the second node is coupled to a secondsignal line to receive a ring signal from the subscriber loop. Thelinefeed circuit includes a signal conversion circuit, which is coupledto the first node and the second node to receive the tip signal and thering signal, respectively. The signal conversion circuit provides acommon mode signal in response to at least the tip signal and the ringsignal. The signal conversion circuit also providing a DC ring signal.The linefeed circuit also includes a filter circuit, which is coupled tothe signal conversion circuit and provides audio band or voice bandsignals. The linefeed circuit also includes a tip control circuitcoupled to the first node and a ring control circuit coupled to thesecond node. The tip control circuit provides a tip output signal to thefirst signal line in response to a first tip control current signal anda second tip control current, whereas the ring control circuit providesa ring output signal to the second signal line in response to a firstring control current signal and a second ring control current signal.

In a specific embodiment, the subscriber line control circuit isprovided in a single integrated circuit chip. In an embodiment, thesubscriber line control circuit (SLCC) includes a common mode input anda DC ring signal input. The common mode input receives the common modesignal from the linefeed circuit, and the DC ring signal input receivesthe DC ring signal from the linefeed input. The SLCC also includes ananalog control circuit coupled to the common mode input and the DC ringsignal input. The analog control circuit provides the first and thesecond tip control current signals and the first and the second ringcontrol current signals, in response to at least the common mode signaland the DC ring signal. The SLCC also includes an amplifier circuit forreceiving audio band or voice band signals from the linefeed circuit,and a codec which coupled to the amplifier circuit.

In an embodiment of the subscriber line interface circuit apparatus, thesubscriber line control circuit (SLCC) also includes an analog todigital converter circuit which is coupled to the common mode input andthe DC ring input. In an embodiment, the SLCC also includes an analogcontrol circuit coupled to the common mode input and the DC ring inputto provide the first tip control current signal, the second tip controlcurrent signal, the first ring control current signal, and the secondring control current signal in response to at least the common modesignal and the DC ring signal. The SLCC also includes several controlsignal outputs. A first and a second tip control outputs are coupled tothe analog control circuit to provide the first and the second tipcontrol current signals, respectively. A first and a second ring controloutputs are coupled to the analog control circuit to provide the firstand the second ring control current signals, respectively.

In a specific embodiment, the common mode input signal is associatedwith a sum of the tip signal and the ring signal. In an embodiment, thesignal conversion circuit further includes a first resistor and a secondresistor. A first terminal of the first resistor is coupled to the firstnode, a second terminal of the first resistor is coupled to the commonmode signal, a first terminal of the second resistor is coupled to thesecond node, and a second terminal of the second resistor is coupled tothe common mode signal. In an embodiment, the tip control circuitincludes a first tip control bipolar transistors and a second tipcontrol bipolar transistor. An emitter terminal of each of the first tipcontrol bipolar transistor and the second tip control bipolar transistoris in communication with the first tip control current signal and thesecond tip control current signal, respectively. In an embodiment, thering control circuit includes a first ring control bipolar transistorand a second ring control bipolar transistor. An emitter terminal ofeach of the first ring control bipolar transistor and the second ringcontrol bipolar transistor is in communication with the first ringcontrol current signal and the second ring control current signal,respectively. In a specific embodiment, the linefeed circuit comprisesdiscrete components, whereas in another embodiment, the linefeed circuitis implemented in an integrated circuit.

Many benefits are provided by way of the present invention overconventional techniques. In a specific embodiment, the inventionprovides a subscriber loop control circuit which includes differentialand common-mode signals, and is relatively isolated from the subscriberloop. There is no need for a signal processor to calculate the commonmode and differential mode signals. In an embodiment, the inventionprovides feedback common mode and differential mode input signals thatallow remote detection of a subscriber line circuit faulty condition asindicated by a melted fuse. For example, by comparing the common modevoltage and differential mode voltages against target voltages, thesubscriber line control circuit can determine whether the fuse betweenthe subscriber loop and the linefeed circuit is melted. This remotefault detection capability can reduce the cost of a subscriber lineservice. In some embodiments, the invention provides a subscriber linecontrol circuit that is relatively less expensive than conventionalcircuits. Embodiments of the invention have a broad range ofapplicability. For example, certain embodiments of the invention areuseful in packet switched telephony applications, which use a ‘plain oldtelephone’ interface. In an embodiment, the invention can be used inresidential VOIP. A specific embodiment of the invention can be used ina device called an ATA (analog terminal adapter), which has an interfaceto the ‘plain old telephone. The above-mentioned remote fault detectioncapability can be especially advantageous in reducing the cost ofservice in residential ATA's. In another example, embodiments of theinvention can be used with an Ethernet port to be in communication withan Internet router. These and other benefits will be described in moredetail throughout the present specification and more particularly below.

Various additional objects, features, and advantages of the presentinvention can be more fully appreciated with reference to the detaileddescription and accompanying drawings that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a subscriber line interfacecircuit (SLIC) 100 according to an embodiment of the present invention;

FIG. 2 a is a simplified block diagram of a subscriber line interfacecircuit (SLIC) 200 according to an another embodiment of the presentinvention;

FIG. 2 b is a simplified block diagram of a subscriber line controlcircuit (SLCC) 210 according to an embodiment of the present invention;

FIG. 3 a is a simplified block diagram of a subscriber line interfacecircuit (SLIC) 300 according to yet another embodiment of the presentinvention;

FIG. 3 b is a simplified block diagram of a subscriber line controlcircuit (SLCC) 310 according to another embodiment of the presentinvention; and

FIG. 4 is a simplified block diagram of a subscriber line interfacecircuit (SLIC) 400 according to an alternative embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides a method and device for anintegrated circuit for telecommunication. Merely by way of example, theinvention has been applied to subscriber line control circuits forinterfacing with digital equipment in a central office. But it would berecognized that the invention has a much broader range of applicability.For example, embodiments of the invention can find application incommunication networks such as cable television networks, fiber optic,Ethernet port interface to the Internet, VoIP, and wireless local loop,etc.

As discussed above, conventional subscriber line interface chipsetcombination of a SLIC chip and a CODEC chip have certain limitations. Adisadvantage of this conventional design is that it involves a largenumber of circuit components on the high voltage and high power SLICintegrated circuit. As a result, conventional designs can be relativelyexpensive. Alternative designs have been suggested. For example, atechnique for partitioning the SLIC into multiple integrated circuits ispresented in K. Kawarada et al, “High Voltage Subscriber Line InterfaceLSIs”, IEEE Journal of Solid-State Circuit, Vol. SC-17, No. 6, pp.1144-1149, December 1982. As shown in Kawarada et al FIG. 2, certaincontrol functions of the SLIC are implemented in a CMOS chip (CONT). Ahigh voltage bipolar LSI chip (RT) performs the ringing and testingfunctions. A medium voltage bipolar LSI chip (BSH) performs the batteryfeed, supervision, and hybrid functions. Kawarada et al also includesanother CMOS chip for the CODEC function. Even though the techniquesprovided by Kawarada et al may have some benefits such as moving certaincircuit components to a low voltage CMOS chip, there are manylimitations. For example, a large number of separate integrated circuitsare needed. In addition, signals on the tip and ring lines are directlyfed into the RT and BSH circuits. They are less isolated from the tipand ring lines. As a result, the circuits may be susceptible tovariations in the tip and ring lines.

In another example, in U.S. Pat. No. 6,934,384 by Hein et al, asubscriber loop interface circuit apparatus includes a signal processorhaving sense inputs for a sensed tip signal and a sensed ring signal ofa subscriber loop. The signal processor generates a linefeed drivercontrol signal in response to the sensed signals and the signalprocessor resides on a same integrated circuit die as a codec forbi-directional communication of voiceband data between the analogsubscriber loop and a digital interface of the signal processor. In U.S.Pat. No. 6,934,384, a signal processor 210 can be found in FIG. 2 andsensed tip signals 332 and 334 and sensed ring signals 336 and 338 canbe found in FIG. 3. U.S. Pat. No. 6,934,384 also provides an apparatusincluding a signal processor generating subscriber loop control signalsin response to a sensed tip signal and a sensed ring signal of asubscriber loop, and a linefeed driver portion for driving thesubscriber loop in accordance with the subscriber loop control signals,the linefeed driver portion providing the sensed tip and ring signals.Each of the linefeed driver portion and the signal processor resides onan integrated circuit die, wherein the signal processor resides on asame integrated circuit die as a codec for bi-directional communicationof voiceband data between the analog subscriber loop and a digitalinterface of the signal processor. A signal processor 210 and a linefeeddriver 220 can be found in FIG. 2 of Hein et al. Sensed tip signals 332and 334 and sensed ring signals 336 and 338 can be found in FIG. 3.Bi-direction voiceband data lines 322 324 and 326 and 328 can be foundin FIG. 3 of Hein et al. Even though the techniques provided by U.S.Pat. No. 6,934,384 may have some benefits, there are many limitations.For example, in U.S. Pat. No. 6,934,384 the signal processor senses thetip and ring lines, being less isolated from the tip and ring lines. Asa result, the signal processor may be susceptible to variations in thetip and ring lines. Moreover, in U.S. Pat. No. 6,934,384, the commonmode and differential mode components are calculated by the signalprocessor rather than the linefeed driver. A disadvantage of this designis that the signal processor requires additional circuitry and powerconsumption. This tends to increase the cost of the design andmanufacture.

Certain other conventional designs appear to use similar techniques asU.S. Pat. No. 6,934,384. For example, in U.S. Pat. No. 7,180,999 by Heinet al and U.S. Pat. No. 7,190,785 by Hein et al, a signal processor 210can be found in FIG. 2 and sensed tip signals 332 and 334 and sensedring signals 336 and 338 can be found in FIG. 3 in the respectivepatent. In U.S. Pat. No. 7,158,633 by Hein, a signal processor 210 canbe found in FIG. 2 and sensed tip signals 332 and 334 and sensed ringsignals 336 and 338 can be found in FIG. 3. Further, in FIG. 7 of U.S.Pat. No. 7,158,633 by Hein, signal processor 710 receives two sensed tipsignals 732 and 734 and two sensed ring signals 736 and 738. Theseconventional designs tend to suffer from similar limitations asdescribed above.

Therefore, improved techniques for subscriber line interface circuit aredesired.

According embodiments of the present invention, improved techniques forsubscriber line interface circuit are provided. Depending upon theembodiment, the present invention includes various features, which maybe used. These features include the following:

-   -   1. A subscriber line interface circuit apparatus for a        subscriber loop that includes a linefeed circuit in combination        with subscriber line control circuitry (SLCC);    -   2. A linefeed circuit that includes a unique signal conversion        circuit which provides both a differential mode signal and a        related common mode signal in response to at least the tip        signal and the ring signal so the SLCC circuitry need not derive        such signals;    -   3. A subscriber line control circuit (SLCC) which receives the        common mode and differential mode signals. The common mode and        differential mode signals can be used directly for monitoring        the common mode and differential mode voltages on the subscriber        line;    -   4. A subscriber line control circuit (SLCC) that is isolated        from the tip signal and the ring signal by the linefeed circuit.        The SLCC is therefore relatively independent of the        instabilities in the tip signal and the ring signal; and    -   5. A subscriber line interface circuit apparatus in which a        linefeed circuit provides common mode and differential mode        signals which can be used for remote detection of a faulty        condition on the subscriber line.

As shown, the above features may be in one or more of the embodiments tofollow. These features are merely examples, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize many variations, modifications, and alternatives. It isalso noted that in this application, the term “subscriber line” and theterm “subscriber loop” are used interchangeably.

FIG. 1 is a simplified block diagram of a subscriber line interfacecircuit (SLIC) 100 according to an embodiment of the present invention.This diagram is merely an example, which should not unduly limit thescope of the claims herein. One of ordinary skill in the art wouldrecognize other variations, modifications, and alternatives. As shown,SLIC circuit 100 includes a subscriber loop control circuit (SLCC) 110and a linefeed circuit 120. In an embodiment, the subscriber loopcontrol circuit 110 provides control signals 126 to the linefeed circuitin response to common mode and/or differential subscriber loop signals122 from the linefeed circuit. In a specific embodiment, the common modeand/or differential mode signals are provided by the linefeed circuit120 and do not need to be calculated by the subscriber loop controlcircuit 110. Therefore, they can be used directly in the subscriber loopcontrol circuit 110 for monitoring the common mode and differential modevoltages on the subscriber line. In an embodiment, a common modesubscriber loop signal can be either a voltage or a current. Forexample, a common mode subscriber loop signal can be related to a sum ofthe subscriber loop voltages. In a specific embodiment, a common modesubscriber loop signal is proportional to a sum of the subscriber loopvoltages, e.g. Tip signal 133 and Ring signal 135. In an embodiment, thedifferential subscriber loop signals can either be a voltage or acurrent. In another example, the differential subscriber loop signal canbe related to the subscriber loop signals, e.g. Tip signal 133 and Ringsignal 135. In a specific embodiment, the differential subscriber loopsignal can be proportional to the voltage across the subscriber loop.

According to an embodiment of the invention, a subscriber line interfacecircuit (SLIC) is provided that includes a subscriber loop controlcircuit (SLCC) and a linefeed circuit. In an embodiment, the SLCC isprovided in a single integrated circuit chip. In a specific embodiment,the SLCC and the linefeed circuit can be built within a singleintegrated circuit package. In another embodiment, the SLCC and thelinefeed circuit can be in separate IC packages. In an alternativeembodiment, the linefeed circuit is implemented using discretecomponents. In an embodiment, the interface between a linefeed circuitand a subscriber loop control circuit can include a common mode and/or adifferential mode subscriber loop signal and voice band or audio banddata signals. In some embodiments, audio band includes a frequency rangeof about 20 Hz to 20,000 Hz and voice band has a frequency range ofabout 300 Hz to 3,400 Hz.

Referring to FIG. 1, in a specific embodiment of the invention,subscriber loop control circuit 110 receives common mode anddifferential mode signals 122 and voice band and/or audio band signals124 from the linefeed circuit 120. Subscriber loop control circuit 110generates tip/ring control signals 126 which include superimposedaudio/voice band signals, in response to at least the commonmode/differential mode signals, the voice band and/or audio bandsignals, the control interface 105 and the Voice band and/or audio bandinterface 107. In an embodiment, the control interface 105 and the Voiceband and/or audio band interface 107 are coupled to a digitalcommunication network. Merely as examples, the digital communicationnetwork can be found in a central office of a telephone serviceprovider, cable television networks, fiber optic, Ethernet portinterface to the Internet, VoIP, and wireless local loop, etc. In aspecific embodiment, the invention provides a subscriber loop controlcircuit which includes a programmable CODEC, a programmable hybrid andprogrammable subscriber line impedance matching for worldwidedeployment. In another embodiment, the invention provides a combinationof a subscriber loop control circuit and linefeed circuit, whichprovides substantially all the BORSCHT functions required for subscriberline operation. Of course, there can be other variations, modifications,and alternatives.

FIG. 2 a is a simplified block diagram of a subscriber line interfacecircuit (SLIC) 200 according to another embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize other variations, modifications, and alternatives. Asshown, SLIC circuit 200 includes a subscriber loop control circuit(SLCC) 210 and a linefeed circuit 220. The linefeed circuit 220 includesTIP control circuit 221, RING control circuit 222, a signal conversioncircuit 223, and a filter circuit 224. In an embodiment, linefeedcircuit 220 includes circuit nodes N_(TIP) and N_(RING). Node N_(TIP) iscoupled to a signal line to receive a tip signal TIP from the subscriberloop. Node N_(RING) is coupled to a second signal line to receive a ringsignal RING from the subscriber loop. In a specific embodiment, thelinefeed circuit 220 also includes a fuse FT coupled between nodeN_(TIP) and tip signal TIP from the subscriber loop, and a second fuseFR and coupled between node N_(RING) and ring signal RING from thesubscriber loop. In an alternative embodiment, fuses FT and FR can beomitted.

As shown in FIG. 2 a, signal conversion circuit 223 provides a commonmode signal S_(COM) and a differential mode signal S_(DIFF) in responseto the tip signal TIP and the ring signal RING of the subscriber loop.In a specific embodiment, the signal conversion circuit 223 includes aresistor R13 coupled between node N_(TIP) and common mode signalS_(COM), and a second resistor R14 between node N_(RING) and common modesignal S_(COM). In a specific embodiment, the common-mode signal S_(COM)is proportional to a sum of the subscriber loop tip and ring voltages.In an embodiment, signal conversion circuit 223 includes transistors QD1and QD2, and resistors RD1, RD2, RD3, and RD4 as shown in FIG. 2 a. Inan embodiment, the differential mode signal is provided across a firstdifferential signal line SD1 and a second differential signal line SD2as shown in FIG. 2 a. In a specific example, transistors QD1 and QD2 arebipolar transistors. As shown, resistor RD4 is coupled between a baseterminal of QD1 and node N_(TIP), and resistor RD3 is coupled between abase terminal of QD2 and node N_(RING). In a specific embodiment,resistor RD2 is coupled between an emitter terminal of QD1 and the ringsignal RING from the subscriber loop, and resistor RD1 is coupledbetween an emitter terminal of QD2 and the tip signal TIP from thesubscriber loop. In the embodiment shown in FIG. 2 a, differential modesignal S_(DIFF) is provided across signal lines SD1 and SD2, which arecoupled to a collector terminal of transistor QD1 and a collectorterminal of transistor QD2, respectively. In an embodiment, differentialmode signal S_(DIFF) is proportional to a difference between the tipsignal and the ring signal. Of course, there can be other variations,modifications, and alternatives. For example, in an alternativeembodiment, the emitter terminal of QD1 and the base terminal of QD2 areboth coupled to node N_(RING), and an emitter terminal of QD2 and a baseterminal of QD1 are both coupled to node N_(TIP).

As shown in FIG. 2 a, linefeed circuit 220 includes a filter circuit224, which includes capacitor CRA and resistor R6 connected in series toprovide an audio band signal input terminal RAC of SLCC 210. Similarly,filter circuit 224 includes capacitor CTA and resistor R4 connected inseries to provide an audio band signal to input terminal TAC of SLCC210. In some embodiments of the present invention, audio band includes afrequency range of about 20 Hz to 20,000 Hz, and voice band has afrequency range of about 300 Hz to 3,400 Hz.

FIG. 2 b is a simplified block diagram of a subscriber line controlcircuit (SLCC) 210 according to an embodiment of the present invention.This diagram is merely an example, which should not unduly limit thescope of the claims herein. One of ordinary skill in the art wouldrecognize other variations, modifications, and alternatives. As shown,the subscriber line control circuit 210 includes amplifier circuit 209,impedance matching circuit 211, hybrid circuit block 212, andprogrammable codec 213. In an embodiment, SLCC 210 also includes analogcontrol circuit 208, digital-to-analog converter circuit (DAC) 217,analog-to-digital circuit (ADC) 218, and logic circuit 219. In aspecific embodiment, analog control circuit 208 includes a summationcircuit 214, analog voltage feedback circuit 215, and current feedbackcircuit 216. In an embodiment, the subscriber line control circuit 210includes input terminals RAC and TAC for receiving voice band or audioband signals from the linefeed circuit 220. In some embodiments, thesecircuit blocks together implement many of the BORSCHT functions. Forexample the amplifier 209 receives AC voice band or audio band signalsfrom the linefeed circuit 220. The impedance matching circuit 211matches the characteristic impedance of the subscriber line. In anembodiment, the hybrid circuit 212 performs 2-wire to 4-wire conversion.The codec 213 performs coding and decoding functions for the voice oraudio band signals. In a preferred embodiment, the SLCC is provided in asingle integrated circuit chip. Of course, there can be othervariations, modifications, and alternatives.

In a specific embodiment, SLCC 210 includes an input SCM for receivingthe common mode signal S_(COM) from the linefeed circuit, and alsoincludes inputs SD1 and SD2 for receiving the differential mode signalS_(DIFF) from the linefeed circuit. As shown in FIGS. 2 a and 2 b, theSLCC 210 is isolated from the tip signal TIP and the ring signal RING bythe signal conversion circuit 223 in the linefeed circuit. In otherwords, SLCC 210 does not sense the tip signal or the ring signalaccording to embodiments of the present invention. In contrast, SLCC 210receives a differential mode signal and a common mode signal, and can berelatively independent from the instabilities of the tip and ringsignals.

As shown in FIG. 2 b, in an embodiment, SLCC 210 includes outputterminals TPP and TPN for providing tip control current signals.Similarly, SLCC 210 includes output terminals RIP and RIN for providingring control current signals. In a specific embodiment, the subscriberline control circuit 210 also includes terminals VBT and VBR forproviding tip and ring control voltage signals, respectively. These tipand ring control current and voltage signals are provided by the analogcontrol circuit 208 in response to at least the common mode signalS_(COM) and differential mode signal S_(DIFF) from the linefeed circuit220, and signals derived from the control interface 205 and voice/audioband interface 207. In an embodiment, the control interface 205 and theVoice band and/or audio band interface 207 are coupled to a digitalcommunication network. For example, the digital communication networkcan be found in a central office of a telephone service provider, cabletelevision networks, fiber optic, Ethernet port interface to theInternet, VoIP, or wireless local loop, etc. Of course, there can beother variations, modifications, and alternatives.

Referring back to FIG. 2 a, in a specific embodiment, the tip controlsignals TPP and TPN are received by tip control circuit 221, which iscoupled to the N_(TIP) node for controlling the TIP signal line. The tipcontrol circuit 221 includes transistors QT1, QT2, and QT3, capacitorCT1, and resistors RT1, RT2, and RT3. In an embodiment, the tip controlsignals TPP and TPN are received at an emitter terminal of transistorsQT1 and QT2, respectively. The tip control circuit 221 is also coupledto capacitors CTL for line capacitance compensation. Resistor R11 iscoupled between output terminal TVE and an emitter of QT3 in the tipcontrol circuit 221. In an embodiment, the subscriber line controlcircuit 210 includes output terminals RIP and RIN for providing ringcontrol signals. In a specific embodiment, the ring control signals RIPand RIN are received by ring control circuit 222, which is coupled tothe N_(RING) node for controlling the RING signal line. In anembodiment, ring control circuit 222 includes transistors QR1, QR2, andQR3, capacitor CR1, and resistors RR1, RR2 and RR3. In an embodiment,the ring control signals RIP and RIN are received at an emitter terminalof transistors QR1 and QR2, respectively. The ring control circuit 222is also coupled to capacitors CRL for line capacitance compensation.Resistor R12 is coupled between output terminal RVE and an emitter ofQR3 in the ring control circuit 222. In an embodiment, SLCC 210 includesa terminal BAT for communication with a VBAT signal through resistorR10. In an embodiment, VBAT is a supply voltage for the high voltagelinefeed circuit. In an example, VBAT can be provided by an externalpower source. In another example, the SLCC includes an on-chip DCDCcontroller which can be used to control VBAT. As shown in FIG. 2 a, VBATis connected to resistors RR2, RR3, RT2, RT3, and R10, and capacitorsCR1 and CT1. Of course, there can be other variations, modifications,and alternatives.

As discussed above, in a specific embodiment, the subscriber linecontrol circuit 210 also includes terminals VBT and VBR, which providevoltage signals to tip control circuit 221 and ring control circuit 222,respectively. As shown in FIG. 2 a, a base terminal of linefeedtransistors QT1 receives a voltage control signal from terminal VBT ofthe SLCC 210. Similarly, a base terminal of linefeed transistors QR1receives a second voltage control signal from terminal VBR of the SLCC210. In an embodiment, the currents through transistors QR1 and QT1 areproportional to the voltage across resistors RRE or RTE, respectively.In a specific embodiment, resistors RRE or RTE are coupled to areference voltage VREF. In a specific embodiment, VREF is an externalreference voltage. In such embodiment, the currents through transistorsQR1 and QT1 are supplied predominantly by the external reference voltageVREF, whereas voltage signals VBR and VBT from SLCC 210 are used tocontrol the current through transistors QR1 and QT1, respectively. Thisspecific embodiment offers many advantages. For example, the SLCC doesnot need to supply large currents to the linefeed circuit. Inconventional design, these current may be in the order of 20-80 mA.According to an embodiment of the present invention, current consumptionof the integrated circuit is reduced and large integrated drivercircuits can be avoided. As a result, integrated circuit heat sinks arenot needed. In an example, VREF can be derived from a power supplyvoltage. In a particular example, VREF is equal to the supply voltage inmagnitude. Of course, there can be other variations, modifications, andalternatives.

As discussed above, in a specific embodiment, fuses FT and FR areprovided in the linefeed circuit 220 as shown in FIG. 2 a. A faultycondition in the subscriber loop can cause either or both fuses to beblown. In an embodiment, the present invention provides a method forremote detection of the faulty condition using the signal conversioncircuit 223. As shown in FIG. 2 a, nodes N_(TIP) and N_(RING) receivetip and ring control signals from the tip control circuit 221 and ringcontrol circuit 222, respectively. Under a typical operating condition,the signals at nodes N_(TIP) and N_(RING) are coupled to the TIP andRING signal lines of the subscriber loop. If either or both of the fusesFT and FR are blown, the TIP or RING lines of the subscriber loop cannot receive these signals. In a specific embodiment, the signalconversion circuit 223 provides common-mode and differential modefeedback signals S_(COM) and S_(DIFF) which can be used to determinewhether either or both fuses FT and FR are blown. In this embodiment,resistor RD4 is coupled between a base terminal of transistor QD1 andnode N_(TIP), and resistor RD2 is coupled between an emitter terminal ofQD1 and the RING signal line on the outside of fuse FR. Similarly,resistor RD3 is coupled between a base terminal of transistor QD2 andnode N_(RING), and resistor RD1 is coupled between an emitter terminalof QD2 and the TIP signal line on the outside of fuse FT. Under a faultycondition, for example, if fuse FT is blown, the emitter terminal oftransistor QD2, coupled to the blown fuse FT through resistor RD1, doesnot receive the expected signal at node N_(TIP), causing an erroneousdifferential mode signal S_(DIFF). On the other hand, the expectedcommon-mode signal S_(COM) associated with the signals on nodes N_(TIP)and N_(RING) is still available through resistors R13 and R14. Byexamining the common-mode and differential mode signals, the SLCC 210can determine that the TIP line of the subscriber loop has a faultycondition. Similarly, a faulty condition in the RING line of thesubscriber loop can also be detected.

According to embodiments of the present invention, subscriber lineinterface circuit (SLIC) 200 can be implemented in various circuitconfigurations. For example, on a specific embodiment, subscriber linecontrol circuit (SLCC) 210 is implemented in an integrated circuit, andlinefeed circuit 220 may be implemented using discrete components. Inanother embodiment, the SLCC is implemented in an integrated circuit andthe linefeed circuit is implemented in a second integrated circuit. Forexample, the SLCC can be implemented in a CMOS integrated circuit. Inanother example, the linefeed circuit can be implemented in a bipolarintegrated circuit. In an alternative embodiment, the SLCC and thelinefeed circuit are implemented in a single integrated circuit, e.g. aBiCMOS integrated circuit. Of course, there can be other variations,modifications, and alternatives.

Although the above has been shown using a selected group of componentsfor the microphone interface circuit, there can be many alternatives,modifications, and variations. For example, some of the components maybe expanded and/or combined. Other components may be inserted to thosenoted above. Depending upon the embodiment, the arrangement ofcomponents may be interchanged with others replaced. Further details ofthese components are found throughout the present specification and thedrawings.

FIG. 3 a is a simplified block diagram of a subscriber line interfacecircuit (SLIC) 300 according to another embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize other variations, modifications, and alternatives. Asshown, SLIC circuit 300 includes a subscriber loop control circuit(SLCC) 310 and a linefeed circuit 320. The linefeed circuit 320 includesTIP control circuit 321, RING control circuit 322, a signal conversioncircuit 323, and a filter circuit 324. In an embodiment, linefeedcircuit 320 includes circuit nodes N_(TIP) and N_(RING). Node N_(TIP) iscoupled to a signal line to receive a tip signal TIP from the subscriberloop. Node N_(RING) is coupled to a second signal line to receive a ringsignal RING from the subscriber loop.

As shown in FIG. 3 a, signal conversion circuit 323 provides a commonmode signal S_(COM) and a differential mode signal S_(DIFF) in responseto the tip signal TIP and the ring signal RING of the subscriber loop.In a specific embodiment, the signal conversion circuit 323 includes aresistor R13 coupled between node N_(TIP) and common mode signalS_(COM), and a second resistor R14 between node N_(RING) and common modesignal S_(COM). In a specific embodiment, the common-mode signal S_(COM)is proportional to a sum of the subscriber loop tip and ring voltages.In an embodiment, signal conversion circuit 323 includes transistors QD1and QD2, and resistors RD1, RD2, RD3, and RD4 as shown in FIG. 3 a. Inan embodiment, the differential mode signal S_(DIFF) is provided acrossa first differential signal line SD1 and a second differential signalline SD2 as shown in FIG. 3 a. In a specific example, transistors QD1and QD2 are bipolar transistors. As shown, resistor RD4 is coupledbetween a base terminal of QD1 and node N_(TIP), and resistor RD3 iscoupled between a base terminal of QD2 and node N_(RING). In a specificembodiment, resistor RD2 is coupled between an emitter terminal of QD1and the ring signal RING from the subscriber loop, and resistor RD1 iscoupled between an emitter terminal of QD2 and the tip signal TIP fromthe subscriber loop. In the embodiment shown in FIG. 3 a, differentialmode signal S_(DIFF) is provided across signal lines SD1 and SD2, whichare coupled to a collector terminal of transistor QD1 and a collectorterminal of transistor QD2, respectively. In an embodiment, differentialmode signal S_(DIFF) is proportional to a difference between the tipsignal and the ring signal. Of course, there can be other variations,modifications, and alternatives. For example, in an alternativeembodiment, the emitter terminal of QD1 and the base terminal of QD2 areboth coupled to node N_(RING), and an emitter terminal of QD2 and a baseterminal of QD1 are both coupled to node N_(TIP).

As shown in FIG. 3 a, linefeed circuit 320 includes a filter circuit324, which includes capacitor CRA and resistor R6 connected in series toprovide an audio band signal input terminal RAC of SLCC 310. Similarly,filter circuit 324 includes capacitor CTA and resistor R4 connected inseries to provide an audio band signal to input terminal TAC of SLCC310. In some embodiments of the present invention, audio band includes afrequency range of about 20 Hz to 20,000 Hz, and voice band has afrequency range of about 300 Hz to 3,400 Hz.

FIG. 3 b is a simplified block diagram of a subscriber line controlcircuit (SLCC) 310 according to an embodiment of the present invention.This diagram is merely an example, which should not unduly limit thescope of the claims herein. One of ordinary skill in the art wouldrecognize other variations, modifications, and alternatives. As shown,the subscriber line control circuit 310 includes amplifier circuit 309,impedance matching circuit 311, hybrid circuit block 312, andprogrammable codec 313. In an embodiment, SLCC 310 also includes analogcontrol circuit 308, digital-to-analog converter circuit (DAC) 317,analog-to-digital circuit (ADC) 318, and logic circuit 319. In aspecific embodiment, analog control circuit 308 includes a summationcircuit 314, analog voltage feedback circuit 315, and current feedbackcircuit 316. In an embodiment, the subscriber line control circuit 310includes input terminals RAC and TAC for receiving voice band or audioband signals from the linefeed circuit 320. In some embodiments, thesecircuit blocks together implement many of the BORSCHT functions. Forexample the amplifier 309 receives AC voice band or audio band signalsfrom the linefeed circuit 320. The impedance matching circuit 311matches the characteristic impedance of the subscriber line. In anembodiment, the hybrid circuit 312 performs 2-wire to 4-wire conversion.The codec 313 performs coding and decoding functions for the voice oraudio band signals. In a preferred embodiment, the SLCC is provided in asingle integrated circuit chip. Of course, there can be othervariations, modifications, and alternatives.

In a specific embodiment, SLCC 310 includes an input SCM for receivingthe common mode signal S_(COM) from the linefeed circuit, and alsoincludes inputs SD1 and SD2 for receiving the differential mode signalS_(DIFF) from the linefeed circuit. As shown in FIGS. 3 a and 3 b, theSLCC 310 is isolated from the tip signal TIP and the ring signal RING bythe signal conversion circuit 323 in the linefeed circuit. In otherwords, SLCC 310 does not sense the tip signal or the ring signalaccording to embodiments of the present invention. In contrast, SLCC 310receives a differential mode signal and a common mode signal, and can berelatively independent from the instabilities of the tip and ringsignals.

As shown in FIG. 3 b, in an embodiment, SLCC 310 includes outputterminals TPP and TPN for providing tip control current signals.Similarly, SLCC 310 includes output terminals RIP and RIN for providingring control current signals. These tip and ring control current signalsare provided by the analog control circuit 308 in response to at leastthe common mode signal S_(COM) and differential mode signal S_(DIFF)from the linefeed circuit 320, and signals derived from the controlinterface 305 and voice/audio band interface 307. In an embodiment, thecontrol interface 305 and the Voice band and/or audio band interface 307are coupled to a digital communication network. For example, the digitalcommunication network can be found in a central office of a telephoneservice provider, cable television networks, fiber optic, Ethernet portinterface to the Internet, VoIP, or wireless local loop, etc. Of course,there can be other variations, modifications, and alternatives.

Referring back to FIG. 3 a, in a specific embodiment, the tip controlsignals TPP and TPN are received by tip control circuit 321, which iscoupled to the N_(TIP) node for controlling the TIP signal line. The tipcontrol circuit 321 includes transistors QT1, QT2, and QT3, capacitorCT1, and resistors RT1, RT2, and RT3. In an embodiment, the tip controlsignals TPP and TPN are received at an emitter terminal of transistorsQT1 and QT2, respectively. The tip control circuit 321 is also coupledto capacitors CTL for line capacitance compensation. Resistor R11 iscoupled between output terminal TVE and an emitter of QT3 in the tipcontrol circuit 321. In an embodiment, the subscriber line controlcircuit 310 includes output terminals RIP and RIN for providing ringcontrol signals. In a specific embodiment, the ring control signals RIPand RIN are received by ring control circuit 322, which is coupled tothe N_(RING) node for controlling the RING signal line. In anembodiment, ring control circuit 322 includes transistors QR1, QR2, andQR3, capacitor CR1, and resistors RR1, RR2 and RR3. In an embodiment,the ring control signals RIP and RIN are received at an emitter terminalof transistors QR1 and QR2, respectively. The ring control circuit 322is also coupled to capacitors CRL for line capacitance compensation.Resistor R12 is coupled between output terminal RVE and an emitter ofQR3 in the ring control circuit 322. In an embodiment, SLCC 310 includesa terminal BAT for communication with a VBAT signal through resistorR10. In an embodiment, VBAT is a supply voltage for the high voltagelinefeed circuit. In an example, VBAT can be provided by an externalpower source. In another example, the SLCC includes an on-chip DCDCcontroller which can be used to control VBAT. As shown in FIG. 3 a, VBATis connected to resistors RR2, RR3, RT2, RT3, and R10, and capacitorsCR1 and CT1. Of course, there can be other variations, modifications,and alternatives.

According to embodiments of the present invention, subscriber lineinterface circuit (SLIC) 300 can be implemented in various circuitconfigurations. For example, in a specific embodiment, subscriber linecontrol circuit (SLCC) 310 is implemented in an integrated circuit, andlinefeed circuit 320 may be implemented using discrete components. Inanother embodiment, the SLCC is implemented in an integrated circuit,and the linefeed circuit is implemented in a second integrated circuit.For example, the SLCC can be implemented in a CMOS integrated circuit.In another example, the linefeed circuit can be implemented in a bipolarintegrated circuit. In an alternative embodiment, the SLCC and thelinefeed circuit are implemented in a single integrated circuit, e.g. aBiCMOS integrated circuit. Of course, there can be other variations,modifications, and alternatives.

Although the above has been shown using a selected group of componentsfor the microphone interface circuit, there can be many alternatives,modifications, and variations. For example, some of the components maybe expanded and/or combined. Other components may be inserted to thosenoted above. Depending upon the embodiment, the arrangement ofcomponents may be interchanged with others replaced. Further details ofthese components are found throughout the present specification and thedrawings.

FIG. 4 is a simplified block diagram of a subscriber line interfacecircuit (SLIC) 400 according to yet another embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize other variations, modifications, and alternatives. Asshown, SLIC circuit 400 includes a subscriber loop control circuit(SLCC) 410 and a linefeed circuit 420. The linefeed circuit 420 includesTIP control circuit 421, RING control circuit 422, a signal conversioncircuit 423, and a filter circuit 424. In an embodiment, linefeedcircuit 420 includes circuit nodes N_(TIP) and N_(RING). Node N_(TIP) iscoupled to a signal line to receive a tip signal TIP from the subscriberloop. Node N_(RING) is coupled to a second signal line to receive a ringsignal RING from the subscriber loop. As shown in FIG. 4, signalconversion circuit 423 provides a common mode signal S_(COM) and directcoupled ring signal S_(RDC) in response to the tip signal TIP and thering signal RING of the subscriber loop. In an embodiment, thecommon-mode signal S_(COM) is coupled with the TIP signal throughresistor R13, and S_(COM) is coupled with the RING signal throughresistor R14. In a specific embodiment, the common-mode signal S_(COM)is proportional to a sum of the subscriber loop tip and ring voltages.In an embodiment, direct coupled ring signal SRDC is derived from theRING signal through resistors R15 as shown in FIG. 4. In a specificembodiment, subscriber line control circuit (SLCC) 410 includes an inputRDC configured to receive the direct coupled ring signal S_(RDC) fromthe linefeed circuit. Of course, there can be other variations,modifications, and alternatives.

As shown in FIG. 4, linefeed circuit 420 includes a filter circuit 424,which includes capacitor CRA and resistor R6 connected in series toprovide an AC ring signal to input terminal RAC of SLCC 410. Similarly,filter circuit 424 includes capacitor CTA and resistor R4 connected inseries to provide an AC tip signal to input terminal TAC of SLCC 410. Inan embodiment, the AC tip and ring signals include voice/audio bandsignals.

In a specific embodiment, SLCC 410 includes an input SCM for receivingthe common mode signal S_(COM) from the linefeed circuit and input RDCfor receiving the direct coupled ring signal S_(RDC) from the linefeedcircuit. As shown in FIG. 4, in an embodiment, SLCC 410 includes outputterminals TPP and TPN for providing tip control current signals.Similarly, SLCC 410 includes output terminals RIP and RIN for providingring control current signals. These tip and ring control current signalsare provided by an analog control circuit (not shown) in response to atleast the common mode signal S_(COM) and the direct coupled ring signalS_(RDC) from the linefeed circuit 420. In a specific embodiment, SLCC410 also includes circuit blocks not shown in FIG. 4, such as analogcontrol circuit, analog-to-digital converter, logic circuit, amplifiercircuit, impedance matching circuit, hybrid circuit, and codec circuit.For example the amplifier receives AC voice band or audio band signalsfrom the linefeed circuit. The impedance matching circuit matches thecharacteristic impedance of the signals to and from the linefeedcircuit. The hybrid circuit performs 2-wire to 4-wire conversion. Thecodec performs signal conversion for the voice or audio band. In anembodiment, these circuit blocks together implements many of the BORSCHTfunctions. Of course, there can be other variations, modifications, andalternatives.

In FIG. 4, the tip control signals TPP and TPN are received by tipcontrol circuit 421, which is coupled to the N_(TIP) node forcontrolling the TIP signal line. The tip control circuit 421 includestransistors QT1, QT2, and QT3, capacitor CT1, and resistors RT1, RT2,and RT3. In an embodiment, the tip control signals tip control signalsTPP and TPN are received at an emitter terminal of transistors QT1 andQT2 respectively. The linefeed circuit also includes resistors, R10 andR11, and capacitors CTL for tip line control. In an embodiment, thesubscriber line control circuit 410 includes output terminals RIP andRIN for providing ring control signals. In a specific embodiment, thering control signals RIP and RIN are received by ring control circuit422, which is coupled to the N_(RING) node for controlling the RINGsignal line. In an embodiment, ring control circuit 422 includestransistors QR1, QR2, and QR3, capacitor CR1, and resistors RR1, RR2 andRR3. In an embodiment, the ring control signals RIP and RIN are receivedat an emitter terminal of transistors QR1 and QR2, respectively. Thelinefeed circuit also includes resistors, R10 and R12, and capacitorsCRL for ring line control. In an embodiment, SLCC 410 includes aterminal BAT for communication with a VBAT signal through resistor R10.In an embodiment, VBAT is a supply voltage for the high voltage linedriver. In an example, VBAT can be provided by an external power source.In another example, the SLCC includes an on-chip DCDC controller whichcan be used to control VBAT. As shown in FIG. 4, VBAT is connected toresistors RR2, RR3, RT2, and RT3, and capacitors CR1 and CT1. Of course,there can be other variations, modifications, and alternatives.

According to embodiments of the present invention, a subscriber lineinterface circuit (SLIC), e.g. 400, can be implemented in variouscircuit configurations. For example, on a specific embodiment, asubscriber line control circuit (SLCC), such as 410, is implemented inan integrated circuit and a linefeed circuit, such as 420, may beimplemented using discrete components. In another embodiment, thesubscriber line control circuit is implemented in an integrated circuitand the linefeed circuit is implemented in a second integrated circuit.For example, the subscriber line control circuit can be implemented in aCMOS integrated circuit. In another example, the linefeed circuit can beimplemented in a bipolar integrated circuit. In an alternativeembodiment, the subscriber line control circuit and the linefeed circuitare implemented in a single integrated circuit, e.g. a BiCMOS integratedcircuit. Of course, there can be other variations, modifications, andalternatives.

Although the above has been shown using a selected group of componentsfor the subscriber line interface circuit, there can be manyalternatives, modifications, and variations. For example, some of thecomponents may be expanded and/or combined. Other components may beinserted to those noted above. Depending upon the embodiment, thearrangement of components may be interchanged with others replaced.Further details of these components are found throughout the presentspecification and more particularly below.

It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this applicationand scope of the appended claims.

What is claimed is:
 1. A subscriber line interface circuit apparatus fora subscriber loop, the apparatus comprising: a linefeed circuit, thelinefeed circuit including: a first node, the first node being coupledto a first signal line to receive a tip signal from the subscriber loop;a second node, the second node being coupled to a second signal line toreceive a ring signal from the subscriber loop; a signal conversioncircuit, the signal conversion circuit being coupled to the first nodeand the second node to receive the tip signal and the ring signal,respectively, the signal conversion circuit providing a differentialmode signal and a common mode signal in response to at least the tipsignal and the ring signal, wherein the differential mode signal isassociated with a difference between the tip signal and the ring signal,and the common mode signal is associated with a sum of the tip signaland the ring signal; a filter circuit, the filter circuit being coupledto the signal conversion circuit, the filter circuit providing audioband or voice band signals; a tip control circuit, the tip controlcircuit being coupled to the first node, the tip control circuitproviding a tip output signal to the first signal line in response to afirst tip control current signal and a second tip control currentsignal; and a ring control circuit coupled to the second node, the ringcontrol circuit providing a ring output signal to the second signal linein response to a first ring control current signal and a second ringcontrol current signal; and a subscriber line control circuit, thesubscriber line control circuit being provided in a single integratedcircuit chip, the subscriber line control circuit being coupled to thelinefeed circuit, the subscriber line control circuit being isolatedfrom the first and second signal lines, the subscriber line controlcircuit including: a first and a second differential mode inputs, thefirst and second differential mode inputs receiving the differentialmode signal from the linefeed circuit; a common mode input, the commonmode input receiving the common mode signal from the linefeed circuit;an analog to digital converter circuit, the analog to digital convertercircuit being coupled to the common mode input and the differential modeinputs; an analog control circuit, the analog control circuit beingcoupled to the common mode input and the differential mode inputs, theanalog control circuit providing the first tip control current signal,the second tip control current signal, the first ring control currentsignal, and the second ring control current signal, in response to atleast the common mode signal and the differential mode signal; a firsttip control output and a second tip control output coupled to the analogcontrol circuit, the first tip control output and the second tip controloutput providing the first tip control current signal and the second tipcontrol current signal, respectively; a first ring control output and asecond ring control output coupled to the analog control circuit, thefirst ring control output and the second ring control output providingthe first ring control current signal and the second ring controlcurrent signal, respectively; an amplifier circuit for receiving theaudio band or voice band signals from the linefeed circuit; an impedancematching circuit, the impedance matching circuit being coupled to theamplifier circuit, the impedance matching circuit being configured forthe subscriber line control circuit to match a characteristic lineimpedance of the subscriber loop; a hybrid circuit, the hybrid circuitbeing coupled to the impedance matching circuit; and a codec, the codecbeing coupled to the hybrid circuit.
 2. The subscriber line interfacecircuit apparatus of claim 1 wherein the signal conversion circuitcomprises a first transistor and a second transistor, wherein a firstterminal of the first transistor is coupled to the first node, a secondterminal of the first transistor is coupled to the second node, a thirdterminal of the first transistor is coupled to a first differential modesignal line, a first terminal of the second transistor is coupled to thesecond node, a second terminal of the second transistor is coupled tothe first node, and a third terminal of the second transistor is coupledto a second differential mode signal line, the first and the seconddifferential mode signal lines providing the differential mode signal.3. The subscriber line interface circuit apparatus of claim 2 where inthe signal conversion circuit further comprises a first resistor and asecond resistor, wherein a first terminal of the first resistor iscoupled to the first node, a second terminal of the first resistor iscoupled to the common mode signal, a first terminal of the secondresistor is coupled to the second node, and a second terminal of thesecond resistor is coupled to the common mode signal.
 4. The subscriberline interface circuit apparatus of claim 1 wherein the tip controlcircuit comprises a first tip control bipolar transistor and a secondtip control bipolar transistor, an emitter terminal of the first tipcontrol bipolar transistor being in communication with the first tipcontrol current signal, an emitter terminal of the second tip controlbipolar transistor being in communication with the second tip controlcurrent signal, a base terminal of the first tip control transistorbeing in communication with a base terminal of the second tip controltransistor.
 5. The subscriber line interface circuit apparatus of claim1 wherein the ring control circuit includes a first ring control bipolartransistor and a second ring control bipolar transistor, an emitterterminal of the first ring control bipolar transistor being incommunication with the first ring control current signal, an emitterterminal of the second ring control bipolar transistor being incommunication with the second ring control current signal, a baseterminal of the first ring control transistor being in communicationwith a base terminal of the second ring control transistor.
 6. Thesubscriber line interface circuit apparatus of claim 1 wherein thelinefeed circuit comprises discrete components.
 7. The subscriber lineinterface circuit apparatus of claim 1 wherein the linefeed circuit isimplemented in an integrated circuit.
 8. An integrated circuit apparatusfor controlling a subscriber loop, the integrated circuit apparatuscomprising: a common mode input and a first and a second differentialmode inputs, the common mode input receiving a common mode signal, thefirst and second differential mode inputs receiving a differential modesignal, respectively, each of the common mode signal and thedifferential mode signal being related to a tip signal and a ring signalof the subscriber loop, the common mode signal being related to a sum ofthe tip signal and the ring signal, the differential mode signal beingrelated to a difference between the tip signal and the ring signal, eachof the common mode signal and the differential mode signal beinggenerated outside the integrated circuit apparatus, the integratedcircuit apparatus being isolated from the tip signal and the ringsignal; an analog to digital converter circuit, the analog to digitalconverter circuit being coupled to the common mode input and the firstand second differential mode inputs; an analog control circuit, theanalog control circuit being coupled to the common mode input and thedifferential mode inputs, the analog control circuit providing a firsttip control current signal, a second tip control current signal, a firstring control current signal, and a second ring control current signal,in response to at least the common mode signal and the differential modesignal; a first tip control output and a second tip control outputcoupled to the analog control circuit, the first tip control outputs andthe second tip control output providing the first tip control currentsignal and second tip control current signal, respectively; and a firstring control output and a second ring control output coupled to theanalog control circuit, the first ring control output and the secondring control output providing the first ring control current signal andsecond ring control current signal, respectively.
 9. A subscriber lineinterface circuit apparatus for a subscriber loop, the apparatuscomprising a linefeed circuit and a subscriber line control circuit, thelinefeed circuit including: a first node, the first node being coupledto a first signal line to receive a tip signal from the subscriber loop;a second node, the second node being coupled to a second signal line toreceive a ring signal from the subscriber loop; a signal conversioncircuit, the signal conversion circuit being coupled to the first nodeand the second node to receive the tip signal and the ring signal,respectively, the signal conversion circuit providing a common modesignal in response to at least the tip signal and the ring signal, thecommon mode signal being related to a sum of the tip signal and the ringsignal, the signal conversion circuit also providing a DC ring signal; afilter circuit, the filter circuit being coupled to the signalconversion circuit, the filter circuit providing audio band or voiceband signals; a tip control circuit coupled to the first node, the tipcontrol circuit providing a tip output signal to the first signal linein response to a first tip control current signal and a second tipcontrol current; and a ring control circuit coupled to the second node,the ring control circuit providing a ring output signal to the secondsignal line in response to a first ring control current signal and asecond ring control current signal; and the subscriber line controlcircuit including: a common mode input and a DC ring signal input, thecommon mode input receiving the common mode signal from the linefeedcircuit and the DC ring signal input receiving the DC ring signal fromthe linefeed input; an analog control circuit, the analog controlcircuit being coupled to the common mode input and the DC ring signalinput, the analog control circuit providing the first tip controlcurrent signal, the second tip control current signal, the first ringcontrol current signal, and second ring control current signal inresponse to at least the common mode signal and the DC ring signal; anamplifier circuit for receiving the audio band or voice band signalsfrom the linefeed circuit; and a codec, the codec being coupled to theamplifier circuit; wherein the subscriber line control circuit isprovided in a single integrated circuit chip.
 10. The subscriber lineinterface circuit apparatus of claim 9 wherein the subscriber linecontrol circuit further comprises: an analog to digital convertercircuit, the analog to digital converter circuit being coupled to thecommon mode input and the DC ring input; a first tip control output anda second tip control output coupled to the analog control circuit, thefirst tip control output and the second tip control output providing thefirst tip control current signal and second tip control current signal,respectively; and a first ring control output and a second ring controloutput coupled to the analog control circuit, the first ring controloutput and second ring control output providing the first ring controlcurrent signal and second ring control current signal, respectively. 11.The subscriber line interface circuit apparatus of claim 9 wherein thecommon mode input signal is associated with a sum of the tip signal andthe ring signal.
 12. The subscriber line interface circuit apparatus ofclaim 9 where in the signal conversion circuit further comprises a firstresistor and a second resistor, wherein a first terminal of the firstresistor is coupled to the first node, a second terminal of the firstresistor is coupled to the common mode signal, a first terminal of thesecond resistor is coupled to the second node, and a second terminal ofthe second resistor is coupled to the common mode signal.
 13. Thesubscriber line interface circuit apparatus of claim 9 wherein the tipcontrol circuit comprises a first tip control bipolar transistor and asecond tip control bipolar transistor, an emitter terminal of each ofthe first tip control bipolar transistor and the second tip controlbipolar transistor being in communication with the first tip controlcurrent signal and the second tip control current signal, respectively.14. The subscriber line interface circuit apparatus of claim 9 whereinthe ring control circuit includes a first ring control bipolartransistor and a second ring control bipolar transistor, an emitterterminal of each of the first ring control bipolar transistor and secondring control bipolar transistor being in communication with the firstring control current signal and the second ring control current signal,respectively.
 15. The subscriber line interface circuit apparatus ofclaim 9 wherein the linefeed circuit comprises discrete components. 16.The subscriber line interface circuit apparatus of claim 9 wherein thelinefeed circuit is implemented in an integrated circuit.